The use of hydroprocessing catalysts in the petroleum industry is known and is well documented. Generally, hydroprocessing catalysts encompass both hydrotreating and hydrocracking catalysts. Typically, hydrotreating catalysts are utilized to facilitate removal of organosulphur and nitrogen compounds from refinery feedstocks as a treatment step prior to quality assessment of the final fuel product. Similarly, hydrocracking catalysts may be used in processes for converting gas oils to transportation fuels and for refining lube oils. The cost associated with these catalysts (i.e. the cost of obtaining/purchasing and of using the catalysts) represents the major cost associated with the conversion of primary hydrocarbons to refined fuel products.
Hydroprocessing catalysts generally comprise molybdate and/or tungstate catalysts promoted by nickel and/or cobalt and supported on an inert material, usually gamma alumina. Typically, commercial hydroprocessing catalysts are prepared by supporting the active metal oxides (e.g. MoO.sub.3, WO.sub.3) on .gamma.-Al.sub.2 O.sub.3. This supporting process may involve successive impregnation/calcination steps followed by promotion with CoO or NiO. After loading into a reactor, the catalysts are activated for hydroprocessing operations by a sulphidation step which serves to convert the supported metal oxide-based catalyst to the more stable metal suphide-based (e.g. MoS.sub.2, WS.sub.2) catalyst. During hydroprocessing, the catalyst activity is usually sustained by the presence of organosulphur compounds in the feedstocks. These compounds supply sulphur to the catalyst through hydrogen sulphide (H.sub.2 S) formation.
As the price of crude oil has declined in recent years, petroleum refiners in general and synthetic crude oil producers in particular have been devoting considerable effort toward developing techniques by which process efficiency will be increased and/or overall process costs will be decreased.
Consideration has been given to developing processes for regenerating deactivated or spent hydrotreating catalysts. Bogdanor et al (Ind. Eng. Chem. Prod. Res. Dev. 1986, 25, 220-230) teach that deactivation of hydrotreating process occurs in at least four different ways:
(i) fouling by depositing of coke, ash and metals; PA0 (ii) sintering of the support with consequent loss of surface area; PA0 (iii) agglomeration of active metals; and PA0 (iv) poisoning of active sites on catalysts (usually by metals contained in feedstock).
Of these, it is believed that fouling is the major cause of deterioration in catalyst performance. Fouling involves the deposition of carbonaceous matter on the catalyst surface (usually referred to as coke). This has at least two deleterious results: (a) physical blockage of the catalytically active surface sites and, on a larger scale, (b) plugging of catalyst pores such that diffusion of the feedstock through the pores to the active sites is impeded. Coke fouling of catalytic sites usually occurs via adsorption of certain molecular species (referred to as coking precursors) which are bound strongly to the sites and may be easily polymerized and/or condensed to form large molecular structures.
Excessive operating temperatures may cause sintering and/or agglomeration of hydrotreating catalysts through crystal growth. Sintering is an irreversible phenomenon whereas redispersion of agglomerated metals is practised industrially.
Catalyst poisoning by metals usually involves contamination of the active sites by, for example, lead, arsenic and sodium contained in the feedstock being treated. Catalyst poisoning differs from fouling in that the former represents a chemical interaction of the feedstock material with the catalyst surface, whereas the latter is a physical phenomenon.
Some of the largest producers of synthetic crude oil may be found in Alberta, Canada. Since hydroprocessing operations in synthetic fuels production involves exposing the catalyst to more severe operating regimes than those used in conventional crude oil refining processes, the hydroprocessing catalysts used by synthetic fuel producers may be considered as a higher risk investment. Upgraded bitumen from fluid and delayed coking operations usually requires hydrotreating prior to blending of distillate streams and pipelining as synthetic crude oil. In such a case, the hydrotreating processes may be used to remove sulphur and nitrogen heteroatoms, and to saturate olefins and some aromatics in naphtha and gas oil coker distillates. Alternatively, the catalysts may be used in refinery hydrocracking operations for upgrading synthetic crude gas oils and lube oils. In such a case, more severe reactions conditions are required with the result that the catalysts may experience considerable fouling (i.e. coking) during the operating cycle.
As used throughout this specification, the term "hydroprocessing catalyst" is meant to encompass catalyst which are deactivated by coke fouling. Nonlimiting examples of such catalysts are those used for hydrotreating hydrodesulphurization, hydrodenitrogenation, and hydrogenation of olefins and aromatics), hydrocracking, gasoline reforming, selective hydrogenation of pyrolysis gasolines, and those used as molecular sieves.
Generally, efficient and successful operation of a commercial hydroprocessing unit involves maintenance of maximum feedstock conversion levels throughout the lifetime of the active catalyst. As the catalyst slowly deactivates, the process temperature is systematically ramped upwardly until the catalyst activity is substantially exhausted--i.e. the catalyst is deactivated or spent. If the appropriate precautionary steps are taken, the catalyst will be reversibly deactivated--i.e. it will be fouled. In this type of operating cycle, catalyst lifetime will typically vary from six to eighteen months depending on feedstock composition and operating conditions. Generally, hydroprocessing light petroleum fractions permits a longer catalyst lifetime when compared to hydroprocessing heavy gas oils. The frequency of reactor downtime is related to the activity and/or the lifetime of the catalyst and to the composition of the feedstock material.